(1) Field of the Invention
Treatment of impure, mixed or contaminated liquids or fusible solids for any or all of the purposes of extraction of valuable products, removal of pollutants, concentration of wastes, concentration of solutions, and separation of mixtures into valuable portions, utilizing the principle of liquation. Inasmuch as all of the above-stated objectives involve unmixing or separation, they all are intended to be embraced within the term "separation" for purposes of describing the present invention.
(2) Description of the Prior Art
Two primary methods of thermal separation of constituents in aqueous mixtures are (1) by distillation or evaporation, and (2) freezing, or fusion. In the case of desalination, salt can be extracted by reverse osmosis and electrodialysis. Thermal separation by distillation and evaporation inherently are heat (energy) consumptive. The most common method of separation by crystallization involves direct contact between the mixture and recycled hydrocarbon gases, which are vaporized and condensed, respectively, to freeze the water and then melt the ice crystals after removal of the solute. Again, in this approach, the costs of hydrocarbons, energy consumption, capital investment and operating costs are prohibitive in terms of kilowatt-hours per pound of potable water or of water that is acceptable for discharge into waterways under applicable governmental regulations.
While distillation equipment, multiple effect evaporators, vapor compression units and direct contact crystallization may be economically feasible for shipboard use and small land-based waterworks, their energy consumption renders them virtually unsuitable for purposes of fulfilling the enormous daily effluent requirements of large industrial plants or for desalination in large waterworks projects. In the prior art, U.S. Pat. Nos. 3,349,573 and 3,293,872 (Rowekamp) espouse "solar freezing" for desalination of seawater in frigid regions of the world which have long, cold winters, by exposing water in shallow trays to the extreme cold atmosphere and very rapidly forming crystals of pure water, from which brine solution is washed away. While this application of the old principle of "liquation" may be feasible on a small scale in arctic regions, it is of no practical benefit or consolation for very large scale industrial applications in warmer climates. Furthermore, the Rowekamp conception of very rapid freezing, which is the antithesis of this invention, would not produce large dendritic crystals with large interstices for containment of high concentration solute.
In the field of pure metallurgy, the sciences of crystal growth and liquation have been studied for perhaps centuries. Liquation is the method of separation of the components of a mixture which depends upon the differences in their fusibility, the conditions necessary for their separation being produced either as the result of partial fusion of a solid mixture by heating, or partial solidification of a liquid mixture by cooling. For example, bismuth was at one time freed from accompanying vein stuff by heating the crude ore in externally heated sloping tubes, whereby the metal melted and drained away, leaving the gangue behind. In the smelting of some metals of low melting point, like tin, such metal impurities in the ore as iron and copper are reduced and dissolved in the tin to a large extent at the higher smelting temperature, and when tapped and solidified the crude metal thus contains more of these impurities than corresponds to their solubility at the melting point of tin; thus, upon reheating a little above the melting point tin sweats out, or liquates, leaving the greater part of the impurities in the solid liquation residue.
In the field of crystallization in general, it is known that crystals often develop a dendritic (treelike) morphology, the trunk and arms of which are usually parallel to definite crystallographic directions. This form permits rapid crystal growth because the tips of the dendritic arms are always near parts of the medium which are relatively undepleted in the crystallizing component and unwarmed by the heat of crystallization. Crystal growth, specifically, is the enlargement of crystals at the expense of materials in contact with them and depends on the rate of diffusion of impurities and the rate of heat flow away from the growing crystal surfaces. It follows that the bounding planes of the crystals are perpendicular to the direction of slowest growth and parallel to planes of densest molecular packing. The more rapidly a crystal is grown and the less pure its growth environment, the greater the number and variety of imperfections it acquires, and tending toward formation of many small crystals, or crystallites (polycrystalline). Comparatively slow rates of formation are required to produce large single crystals free of crystallite grains.
A further reference on the subject of crystallization is the following publication in which I was named as a co-author:
"Dendritic Crystallization of Ice from Aqueous Solutions" by Pradeep K. Rohatgi, Surendar M. Jain and Clyde M. Adams, Jr., published in I & EC FUNDAMENTALS Vol. 7, Page 72, February 1968. (Copyright American Chemical Society.)
Notwithstanding prior knowledge of the above principles and finer theoretical aspects of the art (primarily in metallurgical and chemical applications), means for liquation separation which are substantially conservative of energy, and hence applicable to large scale operations such as heavy industrial water purification plants, have not been revealed heretofore in terms according to the present invention.